If it still doesn't work, here is a Youtube link explaining how to go from European software to American one. It's in French but you can likely understand the procedure.
I definitely agree Eagle's Eye, I mostly use the C23 for still photography (that's why I upgraded from H to H+) and don't want to put bottle bottom in front of the lense. You don't really care when shoting video but for still photography, a poor quality filter degrades the optical quality a lot...
This set isn't produced by Freewell anymore but I still look for one for my H+. 5 ND filters ND4, ND8, ND16, ND32, ND64 in 40.5 mm. If somebody has one set that he doesn't use, please let me know.
Thanks
Claude-Alain